The present invention relates to a single crystal pulling crucible applied to an apparatus for pulling semiconductor single crystals by the Czochralski process (the CZ process) and to a producing method thereof.
For producing single crystals of silicon by the CZ process, a quartz crucible for silicon to be fused in the inside thereof and a carbon crucible for receiving the quartz crucible therein and supporting the same from outside have been used hitherto.
In use, the quartz crucible is subjected to the heat of fused silicon to be softened and thus is put into the state of its outside surface being closely contacting with the inner surface of the carbon crucible. If the quartz crucible is cooled in this state, a large stress will be generated in the carbon crucible having a larger coefficient of thermal expansion than the quartz crucible.
Proposals have been made that a single crystal pulling crucible be made of carbon fiber reinforced carbon composite material or C/C composite material having a mechanical strength to withstand the stress and a coefficient of thermal expansion relatively close to that of the quartz crucible, and to easily correspond with increase of size.
Japanese Utility Model Publication No. Hei 3 (1991)-43250 proposed that the crucible comprising a drum portion and a bottom portion be made of the C/C composite material at the drum portion only or over the entirety thereof. It also proposed that the filament windings be used as the producing method to mold the crucible by carbon fibers impregnated with matrix precursor is being helically wound on a crucible-like mandrel.
However, it is hard to wind the carbon fibers helically on the bottom portion having a curved form like a bowl, without any slip caused. This results in the insufficient winding of the bottom portion, and as such could not allow the bottom portion to have a sufficient strength.
Experiments show that the stress is most concentrated on an area extending from the drum portion to the bottom portion, and in the light of the difficulties in making the entire crucible by using the C/C composite material by the filament windings, the modification was proposed that only a boundary portion between the drum portion and the bottom portion be made of the C/C composite material. However, this modified method also uses the filament windings of carbon fibers impregnated with matrix precursors being wound on a crucible-like mandrel by using a parallel winding or a helical winding, in molding the crucible. Although it is easier to produce only the boundary portion between the drum portion and the bottom portion by using the C/C composite material than to produce the entirety of the drum portion and the bottom portion by using the C/C composite material, since the boundary portion between the drum portion and the bottom portion also has a curved part, it was still hard to effect the parallel winding or the helical winding at such a curved part, without any slip caused. As a result of this, those conventional methods have not succeeded in proposing the single crystal pulling crucibles of the C/C composite material having a sufficient strength not only at the drum portion but also at the bottom portion.
Also, the crucible-like mandrel used in the filament windings has the form of a shaft being extended through both ends of the bottom portion and the drum portion at the center thereof. Because of this, even when the entirety of the drum portion and bottom portion is produced by using the C/C composite material, the hole will inevitably remain at the center of the bottom portion, thus requiring to plug that part with another C/C composite material.
In addition, since the C/C composite material is more porous than graphite, there presented a problem of the C/C composite material tending to react with silicon to form SiC readily.
In order to solve these problems involved in the prior art, the invention has been made. It is the primer object of the invention to provide a single crystal pulling crucible of the C/C composite material wherein not only the cylindrical drum portion but also the bowl-like bottom portion are reinforced by the filament windings.
It is the second object of the invention to provide a single crystal pulling crucible of the C/C composite material which is reinforced as a whole, with a part not fully covered by the filament windings being supplemented by using carbon fiber sheets.
It is the third object of the invention to provide a single crystal pulling crucible of the C/C composite material wherein the bowl-like bottom portion has no hole and the entire bottom portion is reinforced.
It is the forth object of the invention to provide a single crystal pulling crucible of the C/C composite material wherein a surface property of the C/C composite material is so improved as to be resistant to forming SiC.
It is the fifth object of the invention to provide the method for easily and reliably producing a single crystal pulling crucible of the C/C composite material wherein no hole is provided and not only the cylindrical drum portion but also the bowl-like bottom portion are reinforced by the filament windings.
It is the sixth object of the invention to provide the method for effectively producing a single crystal pulling crucible of the C/C composite material wherein a hole is provided and not only the cylindrical drum portion but also the bowl-like bottom portion are reinforced by the filament windings.